In recent years, application of plastic materials to optical articles is remarkable, and for example, plastic materials have already been used or are studied to be used for various optical articles such as a panel for a liquid crystal display, a protective film for a color filter, an eyeglass lens, a Fresnel lens, a lenticular lens, a prism lens sheet for a TFT (thin film transistor), an aspherical lens, a coating agent for an optical disk, a core material or a clad material for optical fibers, an optical waveguide, and an adhesive for optical fiber connection.
Plastic materials have been widely used by virtue of their characteristics such that they are easily molded and they are light in weight.
On the other hand, a member for an optical material such as a lens, a prism or a phase element is required to have a high refractive index as an important physical property. For example, in the case of a lens, having a high refractive index is advantageous in that the lens can be downsized or spherical aberration can be reduced. Further, as the base material of the optical material, glass has been well known, however, use of a resin material is desired in view of the weight saving, the economical efficiency, the safety etc. However, a resin material does not have a refractive index as high as glass, and incorporation of inorganic fine particles has been proposed for the purpose of increasing the refractive index, but such may impair the transparency. Accordingly, development of a high refractive index resin only by resin components is desired.
Further, an optical lens such as a prism lens sheet or a Fresnel lens to be used for backlight of a liquid crystal display has a higher transmittance along with e.g. high definition of images in recent years, and its lens shape becomes more complicated and defined, and accordingly its material is desired to have a low viscosity so that handling and forming are easily conducted. Further, in the field of optical communication such as optical fibers and an optical waveguide employing light having a wavelength region of from 650 to 1,600 nm, a resin which is less deformed at the time of curing, that is, which has low shrinkage on curing, has been desired to prevent separation from a substrate and to improve the accuracy of finishing. Further, a cured product having a high glass transition temperature has been desired in order that the shape and the like will not change at the time of use under high temperature environment.
For example, Patent Document 1 discloses that a resin containing a compound having a carbazoyl group is excellent in optical properties such as a high refractive index resulting from the compound having a carbazoyl group, the photorefractive effect, photoconductivity and hole transport property. Particularly in recent years, in view of demands for a high refractive index of plastic materials, the high refractive index of compounds having a carbazoyl group attracts attention, and many attempts to develop a novel highly transparent plastic material are made.
However, in a case where a low molecular weight carbazole compound is used as added to a resin composition, it is hardly soluble in a polymer, and cannot uniformly be mixed in many cases. Further, in a case where it is uniformly mixed forcibly, problems such as bleed out from the resin matrix will occur. Further, N-vinylcarbazole (NVCz) which is a reactive carbazole compound has problems such as low copolymerizability and toxicity in addition to the low solubility. Further, many of carbazoles are solid, and they can hardly be handled as a photocurable resin as they are. Therefore, a plastic material from which a cured product can easily be obtained, of which a cured molded product has a high refractive index, and which can easily be handled, has been desired.
Patent Document 2 discloses an optical material comprising a resin composition containing at least N-vinylcarbazole, polyvinylcarbazole and a photopolymerization initiator, such that crystallization of the optical material can be suppressed in molding at room temperature, the dropping amount suitable for replica molding can properly be controlled, and an optical element with a higher refractive index dispersion is formed by photopolymerization. However, such a resin composition for an optical material has a high viscosity, and application and curing are carried out while heating it at 80° C. Further, vinylcarbazole does not have favorable heat stability, and may form an oligomer component when heated, whereby stable production may not be possible.
Patent Documents 3 and 4 disclose an optical material composition which has an appropriate anomalous dispersion property when formed into a cured product and which is easily processed, and as an optical element using its cured product, a resin composition comprising a (meth)acryloyloxy group-containing compound having a fluorene ring, a compound having at least one (meth)acryloyl group or vinyl group in one molecule and having no fluorene ring, and a polymerization initiator. However, a bisarylfluorene compound used in these documents has a very high viscosity at room temperature and is difficult to handle. Further, due to the high viscosity, packing in a defined shape may be difficult.
Patent Document 5 discloses as a resin composition suitable for an optical lens to be formed on a substrate such as a lenticular lens, a prism lens or a microlens, and as a cured product having excellent transmittance and releasability and having a lower shrinkage on curing, an energy ray-curable resin composition for an optical lens comprising an urethane (meth)acrylate having a bisphenol A skeleton, a polyalkylene oxide-modified bisphenol A di(meth)acrylate and a photopolymerization initiator. However, the shrinkage on curing of the resin composition using bisphenol A type diacrylate is less than 5% and is not yet satisfactory.
Patent Document 6 discloses a resin composition comprising as an essential component a polymer having a polymerizable reactive group suitable for an optical waveguide by virtue of the heat resistance and low shrinkage on curing, however, the shrinkage on curing is about 5%, and the heat resistance is not necessarily sufficient.